Chromated-copper-arsenate (CCA) is widely used to preserve wooden articles, particularly those produced from softwoods such as lodgepole white, jack and red pine. The CCA is applied to the wood under pressure so that it penetrates below the surface of the wood. Under ideal conditions, the CCA bonds with the cellulose by undergoing a chemical reaction (the chromium undergoes a valence change from the hexavalent to the trivalent state) and is rendered insoluble in the wood in a relatively short period of time, after which the CCA does not leach out under normal conditions. Study has suggested that several weeks are required to ensure fixation of CCA on wooden poles under normal temperate climatic conditions. In winter, the fixation period can extend over several months. Environmental contamination at wood preserving plants has been the focus of considerable attention, internationally.
In Canada, Environment Canada has funded a major cooperative project involving the wood treating industry. The project included those responsible for regulating the use of wood preservatives, namely scientists, and health and safety authorities. Key recommendations identified in the document relate to the design of wood preserving facilities in order to minimize potential for environmental contamination. These recommendations include under-cover storage to hold freshly treated wood until fixation of chromated-copper-arsenate (CCA) preservative has been completed. Since the fixation reaction is temperature dependent, long storage times may be anticipated during winter months before CCA treated wood can be released for use.
Current industrial practice for CCA-treated timber and roundwood is to hold the freshly wood under covered storage for a period up to forty eight hours. There is little evidence to indicate that this time period provides an adequate CCA fixing time. If the product is placed in open storage before fixation is complete, loss of preservative components, particularly those at the surface of the wood, will result in serious ground contamination and reduced timber and pole performance in the field.
A potential approach to solving this problem is to accelerate the fixation in some way, such as applying post-treatment heat. Such a procedure could have the added benefit of partially drying the poles, prior to their being placed in outdoor storage. This approach could also be beneficial in that it would reduce the storage time required for the moisture content of the poles to reach levels specified by utility companies prior to delivery. Applying moderate heat to the treated poles could effectively eliminate ground contamination in the storage yard, reduce the storage space requirements and reduce inventory costs.
Unfortunately, other factors such as excessive surface hardness and wood checking characteristics, which also impact on the acceptability of CCA-treated poles, may be detrimentally affected by the heating process.
Certain patents illustrate procedures which are potentially relevant to this field of technology. U.S. Pat. No. 4,716,054 discloses a two-stage process for the accelerated fixing of chromate-containing wood preservative salts in which freshly impregnated timbers are subjected to fixing with superheated steam. The process is carried out by subjecting the freshly impregnated poles to a dry heat treatment in which the wood surface is heated to 60.degree.-100.degree. Celsius, (140.degree. F. to 212.degree. F.) preferably 80.degree.-95.degree. Celsius (180.degree. F.-205.degree. F.). The poles are then treated with superheated steam. The chromated wood preservative salt disclosed is chromium-copper-boron.
Japanese patent No. 70025789 discloses a method of fixing a preservative in wood. The method comprises impregnating wood with a preservative containing chromium, copper and arsenate by pressure or the like and heating the impregnated wood with steam of a temperature less than 150 degrees C. to rapidly fix the preservative in the wood. This Japanese patent does not discuss relative humidity factors which have an important effect in ensuring successful fixing of the chromium-copper-arsenate with the wood.
A publication that may have some relevance to the invention is an article entitled "Fundamentals on Steam Fixation of Chromated Wood Preservatives" by R. D. Peek at al. presented at the Nineteenth Annual Meeting of The International Research Group on Wood Preservation, Madrid, 24-29 April 1988, Spain.